Metallic dielectric photonic crystals and methods of fabrication

ABSTRACT

A metallic-dielectric photonic crystal is formed with a periodic structure defining a plurality of resonant cavities to selectively absorb incident radiation. A metal layer is deposited on the inner surfaces of the resonant cavities and a dielectric material fills inside the resonant cavities. This photonic crystal can be used to selectively absorb broadband solar radiation and then reemit absorbed radiation in a wavelength band that matches the absorption band of a photovoltaic cell. The photonic crystal can be fabricated by patterning a sacrificial layer with a plurality of holes, into which is deposited a supporting material. Removing the rest of the sacrificial layer creates a supporting structure, on which a layer of metal is deposited to define resonant cavities. A dielectric material then fills the cavities to form the photonic crystal.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/478,381, which was filed on Sep. 5, 2014, and which in turn claimspriority, under 35 U.S.C. §119(e), from: U.S. Application No.61/874,405, filed Sep. 6, 2013, and entitled “Wafer scale fabrication of2D nanoscale metallic photonic crystals,” and U.S. Application No.61/905,472, filed Nov. 18, 2013, and entitled “Wafer scale fabricationof 2D nanoscale metallic photonic crystals.” Each of these applicationsis hereby incorporated herein by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under Contract Nos.DE-SC0001299 and DE-FG02-09ER46577 awarded by the US Department ofEnergy. The government has certain rights in the invention.

BACKGROUND

Conventional photovoltaic systems, in which sunlight is directlyconverted into electricity by solar cells, suffer from two majordrawbacks. First of all, they generally stop working after sunset whenthe electricity they are designed to generate is needed most. Thisintermittent nature also makes it difficult to integrate solar energysources into existing power grids. Second, solar cells usually can onlyabsorb light within a narrow band of wavelengths (e.g., in the nearinfrared (IR) band) out of the broadband spectrum of solar radiation,potentially limiting the overall conversion efficiency from opticalenergy to electrical energy.

SUMMARY

One system that addresses the drawbacks of conventional photovoltaicsystems is a solar thermophotovoltaic (STPV) system 100 as shown inFIG. 1. In an STPV system 100, a selective absorber 122 converts thesolar radiation transmitted through a solar concentrator 110 intothermal energy in order to heat up a selective emitter 124, which istuned to re-radiate the thermal energy within a spectral band that canbe efficiently absorbed by one or more solar cells 130. The intermediateconversion step realized by the wavelength selective device(absorber/emitter) 120 not only increases the overall conversionefficiency by matching the re-radiation wavelength to the band gap ofsolar cells, but also enables steady operation of solar power plants bystoring the thermal energy for electricity generation in the nights.

At the heart of an STPV system is the selective absorber 122 in thewavelength selective device 120. Other than the selective absorptionacross the solar spectrum, the selective absorber may possess severalother properties. Typically, a solar concentrator focuses radiation ontothe selective absorber to minimize thermal emission loss. Withincreasing levels of optical concentration, sunlight is delivered atincreasing angles of incidence. Actual optical concentrators alsotypically deliver diffuse-like radiation to the absorber due tonon-idealities and practical design considerations. A selectiveabsorber, therefore, may possess a large acceptance angle for absorptionat wavelengths below a cut-off wavelength (λ_(c)), while maintaining lowhemispherical emissivity above the cut-off wavelength. Furthermore, formaximum efficiency, a STPV absorber is expected to operate attemperatures over 1100 Kelvin, which poses a thermal-stabilitychallenge. Finally, in order to achieve widespread application of STPVsystems, the manufacturing of the selective absorbers can bewafer-scale.

Several methods have been developed to produce selective absorbers. Forexample, metal-based selective absorbers can have a tailored absorptionspectrum. One dimensional metal dielectric stacks have demonstratedpromising solar absorbing properties but tend to be unstable attemperatures greater than 600° C. Two-dimensional metallic air photoniccrystals (MAPhC) can selectively absorb light in the near-IR via cavitymodes and withstand high temperatures greater than 600° C.; however,their acceptance angle is limited to ±300 and the absorption in thevisible spectrum is limited due to diffraction. Metamaterial andplasmonic based absorbers have demonstrated wide angle absorption due totheir sub-wavelength periodic structures, but high temperature stabilityand wafer-scale fabrication have yet to be shown.

Exemplary embodiments of the present technology include apparatus andmethods for selectively absorbing a radiation within a certainwavelength region and converting the radiation energy into thermalenergy to heat up an emitter to re-radiate the energy at anotherwavelength region for electricity generation. Examples of theseapparatus can exhibit selective absorption across a large portion thesolar spectrum, reliability at high temperatures, and large acceptanceangles.

In one example, the apparatus includes a photonic crystal with aperiodic structure defining a plurality of resonant cavities toselectively trap and absorb an incident radiation below a cut-offwavelength that is determined by the wavelength of the optical modessupported by at least one of the resonant cavities. A layer of metal isdisposed on the inner surface of the resonant cavity to improve thereflectivity. The resonant cavity is further filled with a dielectricmaterial to adjust the optical modes to tune the spectral range withinwhich radiation can be effectively absorbed. The dielectric material mayalso help, at least in part, maintain the periodic nanostructure of thephotonic crystal at high temperatures and enlarge the acceptance angleat which the incident radiation can be absorbed. An anti-reflectioncoating may be deposited on the periodic structure to further reduce thereflection loss of the incident radiation energy. The anti-reflectioncoating may comprise the same dielectric materials as filled in theresonant cavities. A diffusion barrier layer may be disposed below theperiodic structure to prevent diffusion or mixing of the metal layer andthe substrate, further enhancing the thermal stability of the photoniccrystal.

In another example, the apparatus includes a solar concentrator toreceive the solar radiation and direct the radiation toward a wavelengthselective device, which is in optical communication with the solarconcentrator and absorbs the solar radiation below a cut-off wavelengthand converts it into thermal energy in order to re-emit it as radiationin a predetermined wavelength band. The wavelength selective device hasa metallic structure defining at least one resonant cavity that isfilled by a dielectric material. An anti-reflection coating is depositedon the metallic structure to further improve absorption efficiency.Following the wavelength selective device is a photovoltaic cell toreceive the re-emitted radiation and convert the radiation intoelectricity. The wavelength band of the re-emitted radiation is tuned tomatch the band gap of the photovoltaic cell in order to optimize theconversion efficiency. The metallic structure may comprise a periodicstructure defining a plurality of resonant cavities and a layer of metaldisposed on the inner surface of the resonant cavities to enhanceabsorption. The resonant cavity of the wavelength selective device maybe designed such that it supports at least one resonant mode with awavelength greater than the cut-off wavelength.

According to another example, a method to fabricate a photonic crystalthat may function as a wavelength selective device is provided. Themethod starts from a substrate coated with a sacrificial layer, which isto be patterned with a plurality of holes arrayed at a period equal toor less than a predetermined wavelength. A supporting material is thendeposited over the inner wall of the holes, followed by the removal ofthe rest portion of the sacrificial layer, leaving only the supportingmaterial to form a supporting structure. The next step is to deposit alayer of metal on the surface of the supporting structure to defineresonant cavities, which are filled with a dielectric material to formthe photonic crystal. The method may further comprise depositing a layerof the dielectric material on top of the resonant cavities to form ananti-reflection coating. The method may also comprise annealing theresonant cavities to improve the absorption. A diffusion barrier layermay be coated on the substrate to prevent the metal layer from mixingwith the substrate and therefore help, at least in part, to maintain thenanostructure of the photonic crystal at high temperatures.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1 shows a STPV system that generates electricity from solarradiation via an intermediate step of selective absorption andreemission.

FIG. 2 shows a cross section view of a selective absorber according toone exemplary embodiment.

FIG. 3 shows a system that converts solar radiation into electricityusing a selective absorber photonic crystal like the one shown in FIG.2.

FIGS. 4A-4K illustrate a process of fabricating a photonic crystal withwavelength selective properties like the one shown in FIG. 2.

FIGS. 5A-5F show scanning electron microscope (SEM) images of thephotonic crystal at different points in the fabrication processillustrated in FIGS. 4A-4K.

FIG. 6A shows a photo of a 6-inch wafer on which several selectiveabsorbers photonic crystals are located.

FIG. 6B shows an SEM image of the cross section of a selective absorberphotonic crystal taken at a 420 angle.

FIG. 7 shows experimental and simulated absorption spectra of a photoniccrystal produced using the process shown in FIGS. 4A-4K and of a flatruthenium layer is also included for comparison.

FIG. 8 shows the experimental absorption spectra of a photonic crystalproduced using the process shown in FIGS. 4A-4K for radiation atdifferent incidence angles.

FIG. 9 shows the experimental absorption spectra of a photonic crystalproduced using the process shown in FIGS. 4A-4K after annealing in anoven at 1000° C. for 24 hours.

FIG. 10 shows the simulated absorption spectra of selective absorberphotonic crystals with anti-reflection coatings of differentthicknesses.

FIG. 11 shows simulated absorption spectra of selective absorberphotonic crystals with supporting structures and metal layers ofdifferent thicknesses.

DETAILED DESCRIPTION

A possible form of an absorber with wavelength-selective properties asused in an STPV system is a two-dimensional photonic crystal thatcomprises a periodic array of resonant cavities. On the microscopicscale, the basic element of selective absorbers is an individualresonant cavity or waveguide, which can effectively trap and absorbradiation of certain wavelength that is in resonance with one of thecavity modes. Therefore, the distribution of cavity modes of eachresonant cavity determines the overall shape of the absorption spectrumof a selective absorber. On the other hand, on the macroscopic scale,the periodic distribution of resonant cavities collectively creates atwo dimensional grating structure, which diffracts incident radiationaccording to classical grating equations. Diffraction induces losses toabsorption because diffraction directs the incident radiation away fromthe selective absorber rather than into it. As a result, gratingparameters, such as period and incident angle, determine the absoluteabsorption percentage of selective absorbers. Due to this cavity-gratingduality, designing a selective absorber with good efficiency may bebased at least in part on theories in both fields.

Without being bound by any particular theory, it appears that theselective absorption of a selective absorber is controlled at least inpart by the cut-off wavelength of the cavity modes. For incident lightwith free-space wavelength greater than the cut-off wavelength of thecavity (λ_(c)), the incident light has no supported cavity mode tocouple into, and the light is thus reflected. Incident light with afree-space wavelength less than the cut-off wavelength of the cavity(λ_(c)) couples into the cavity modes and thus absorption is enhanceddue to the increased interaction time with the walls. For illustrativepurpose only, the cavity modes for a cylindrical waveguide can beanalytically calculated by:

$\begin{matrix}{\lambda_{mn} = \frac{2{\pi\left( {r + {\delta\left( \lambda_{m,n} \right)}} \right)}n}{\chi_{mn}^{\prime}}} & (1)\end{matrix}$where λ_(mn) is the free-space wavelength of the cavity mode (m, n), ris the radius of the cavity, δ(λ_(mn)) is the skin depth as a functionof wavelength, n is the refractive index of the material in the cavity.χ′_(mn) is the root of the derivative of Bessel function of the firstkind (TE fundamental mode). For typical STPV systems, the cut-offwavelength is set to the band gap wavelength of the photovoltaic cell,and is thus a predetermined constant. As a result, the cavity radius andthe refractive index can be adjusted to match a particular cut-offwavelength. By choosing a cavity index n>1, the radius of the cavity canbe reduced by a factor proportional to n.

Efficient coupling of incident lights at oblique angles into cavitiesmay be achieved by reducing or minimizing diffraction losses. Withoutbeing bound by any particular theory, diffraction losses may allowfree-space radiation to reflect into undesired diffraction channelsinstead of coupling into the cavity modes. An analysis of diffraction inthe case of the grating structure of a selective absorber can begin withthe grating equation:a(sin θ_(i)+sin θ_(m))=mλ,m=±1,±2,±3 . . .  (2)where a is the period, θ_(i) is the incident angle of the light, θ_(m)is the diffractive angle of order m, and λ is the free-space wavelengthof light. The onset of diffraction at order m for normally incidentlight (θ_(i)=0°) occurs when θ_(m)=90° which leads to a=mλ. For light atoblique angles, the onset of diffraction at order m occurs at:θ_(i)=sin⁻¹(λm/a±1)  (3)

According to Equation (3), larger ratio of λ/a leads to larger value ofθ_(i), thereby allowing larger incident angles of light impinge on theselective absorber without causing diffractive losses. Note that theperiod can be a factor of 2 smaller than the free space wavelength forθ_(i) to be 90° at the m=1 mode (the cut-off mode).

As previously discussed, for STPV systems, the cut-off wavelength λ_(c)is usually determined at least in part by the band gap wavelength of thephotovoltaic cell. One way to reduce the period a without changing thecut-off wavelength λ_(c) is to increase the index of the cavity n. Forexample, using Equation (1) with an ideal metal where δ(λ_(mn))=0, withan air filled cavity of n=1, radius r=586 nm, period a=2r+100 nm,λ_(c)=2 μm, the ratio λ_(c)/a=1.57, which yields a maximum incidentangle of 34.8°. However, if the cavity is filled with a dielectricmaterial with n=1.3, the radius reduced to r=450 nm, λ_(c)=2 μm, thenthe ratio λ_(c)/a=2 which yields a maximum incident angle of 90°. Thuseven a small increase in the cavity refractive index can increasesignificantly the incident angle without incurring diffraction losses atthe cut-off wavelength. To avoid diffraction losses for higher ordermodes, higher values of the cavity index is preferred.

A Resonant Cavity in a Metallic-Dielectric Selective Absorber

As one possible form of a selective absorber, a photonic crystal maycomprise a periodic array of resonant cavities with wavelength selectiveproperties. FIG. 2 shows a cross-section view of a resonant cavity 200in a periodic structure (not shown in FIG. 2) of a selective absorberaccording to one exemplary embodiment. The resonant cavity 200 includesa supporting wall 210 that has an inner surface 212, a metal layer 220deposited on the inner surface 212, and a dielectric material 230 filledinside the resonant cavity. The outer surface 214 of the supporting wall210, as well as the bottom 216 and top 218 of the cavity may also becoated with a metal layer 220. The dielectric material 230 may also fillpossible spaces between cavities as shown in FIG. 2.

The resonant cavity 200, in this particular embodiment, is cylindricalfor the simplicity of theoretical analysis of cavity modes, but it maybe of any shape including, among others, square, elliptical,rectangular, triangle, or any other polygon. The shape may even beirregular if a particular optical mode is desired. The material used toform the wall 210 of the resonant cavity may be compatible withconformal deposition via Atomic Layer Deposition (ALD) or otherprocesses known in the art, and can be etched via reactive ion etching(RIE). The material may also be compatible with complementarymetal-oxide-semiconductor (CMOS) processes and should have sufficientmechanical strength to stand by itself and support the metal layer 220deposited on it. In one example, Alumina Oxide (Al₂O₃) is employed toform the supporting wall 210 that defines the resonant cavity 200. Inother examples, HfO₂ or TiO₂ may also be used to form the supportingwall 210.

In operation, the metal layer 220 deposited on the inner surface of theresonant cavity reflects the radiation inside the resonant cavity toestablish optical modes. The reflectivity of the metal layer 220 can beover 70% at wavelengths of interest (for example, 0.5 μm-2 μm), but itmay be lower than 70% if, for example, the resonant cavity supports thedesired optical mode(s). Wavelengths of interest may be the cut-offfrequency as in an STPV system, or a desired transmission wavelength inan extraordinary optical transmission system. Materials that can be usedfor the metal layer 220 include tungsten, ruthenium, platinum, silver,tantalum, copper, gold and titanium, among others. However, for certainapplications, non-metal materials such as silicon may also be used hereas long as the reflectivity is sufficient.

For each wavelength of interest, there is a skin depth corresponding toeach material used for the metal layer 220. As readily understood bythose of skill in the art, the skin depth 6 represents the depth towhich radiation penetrates a given material according to Equation (4):δ≈2ρ√{square root over (∈/μ)}  (4)where ρ, ∈, and μ are the resistivity, the permittivity and thepermeability of the material, respectively. The thickness of the metallayer 220 should be greater than the skin depth of the wavelength ofinterest in order to clearly define optical modes. If, however, thethickness of the metal layer 220 is less than the skin depth, incidentradiation may penetrate through the metal layer and reach the supportingwall 210, which also defines a cavity and corresponding optical modes.In this case, the supporting wall 210 can alter the overall absorptionspectra by supporting propagation of additional optical modes within thewall. By tuning the combination of the cavity wall and the metal layer,the desired absorption spectra can be achieved.

In operation, the dielectric material 230 inside the resonant cavity mayfulfill one or more of the following purposes. First of all, thedielectric material 230 adjusts the refractive index of the resonantcavity and thereby tunes the optical modes inside the cavity accordingto Equation (1). Moreover, combinating Equations (1) and (3)demonstrates that the dielectric material 230 allows wide angleoperation of the selective absorber by reducing the period withoutchanging the cut-off wavelength, which is usually a predetermined valuein a STPV system. The dielectric material 230 also improves the overallmechanical stability of the selective absorber. At high temperatures,for example, periodic arrays of empty cavities suffer from a series ofcurvature dependent degradations including surface diffusion,evaporation, recondensation and oxidation. Filling the cavities withdielectric material 230 creates an optically patterned but geometricallyflat surface, thereby avoiding the degradation without compromising theoptical function of the device.

The dielectric material 230 may be substantially transparent (forexample, >90% transmission through the depth of the cavity) to radiationat wavelengths of interest. For example, if used in an STPV system, thenthe dielectric material 230 may transmit in the infrared region of theelectromagnetic spectrum. Furthermore, the refractive index of thedielectric material 230 may be larger than one in order to increase theoperating angle of the selective absorber. The operating angle of aselective absorber may be defined as the largest incident angle at whichthe radiation of interest can still be absorbed by more than 70%. It isalso beneficial to take into account the thermal expansion coefficient(TEC) of the dielectric material 230 and the surrounding metal layer 220when designing the resonant cavities. While ramping up to high operatingtemperatures, if the dielectric material 230 expands more than the metallayer 220, a compressive stress across the dielectric/metal layer iscreated, thus mechanically securing the filling and improving thethermal stability. This can be achieved by selecting a dielectricmaterial with a TEC smaller than that of the surrounding metal material.Materials that exhibit these mechanical, optical, and thermal propertiesinclude, but are not limited to, HfO₂, SiO₂, TiO₂, Al₂O₃, TiN, and otheroxide ceramics. Note that for some applications, not all of theseproperties are necessary. For example, if room temperature operation isdesired, then the TEC consideration can be ignored.

The performance of a selective absorber may also depend on thedimensions of the cavity and the various layers. On a microscopic scale,the radius and depth of the cavity, as well as the thickness of the walland the metal layer, can affect the performance of the selectiveabsorber by altering the optical modes. According to Equation (1), thewavelength of optical modes is a function of both the radius and therefractive index of the dielectric material 230. The radius can lie in awide range from about 50 nm to about 5 μm because one can adjust therefractive index inside the cavity to fit the predetermined cut-offwavelength. The depth of the cavity can be greater than half of theperiod of the periodic structure, but a smaller depth is also feasiblesince other parameters can compensate or balance effects induced by ashallow depth. The thickness of the metal layer, as described before,can be greater than the skin depth at the wavelength(s) of interest, butmay be smaller than the skin depth as well since the cavity wall canalso trap radiation and alter the optical modes inside the cavity.Experimentally, a thickness greater than 50 nm is sufficient toestablish desired optical modes and achieve desired absorption spectrum.The thickness of the cavity wall may be based on mechanicalconsiderations. As long as the wall is strong enough to stand by itselfand support the metal layers deposited on it, the thickness is good. Apractical range would be between 10 nm to 200 nm.

Turning to the macroscopic picture of the selective absorber, the devicecan be designed from a grating point of view. In one example, the periodof the selective absorber is selected to be less than a predeterminedcut-off wavelength to support at least one optical mode whose wavelengthmatches the selective absorber's cut-off wavelength. The cut-offwavelength can be, for example, a band gap wavelength of a photovoltaiccell. A practical range of this cut-off wavelength can be from about 0.5m to about 3 m. In another example, the period of the selective absorberis selected to be less than half of a predetermined cut-off wavelengthin order to reduce or minimize the diffraction loss at large incidentangles. In this particular embodiment, the selective absorber caneffectively absorb incident radiation at an incident angle from 0°(normal incidence) to about 70° (close to grazing incidence).

Note that for STPV applications, the selective absorber, in the form ofa photonic crystal, can be designed with reference to a cut-offwavelength, which may be further selected based on the band gapwavelength and/or the operating temperature of a photovoltaic cell. Inother applications, including but are not limited to plasmonics andextraordinary optical transmission, the photonic crystal may be designedusing other physical quantities as the reference such as the desiredemission direction of beams.

In one aspect, the dielectric-metal interface 290 (for example, FIG. 4I)is directly exposed to the incident radiation. In another aspect (forexample, FIG. 2 or FIG. 4H), an anti-reflection coating 240 is depositedon top of the periodic structure to reduce reflection losses and furtherincrease the absorption. The anti-reflection coating 240 can be a singlelayer film or a multi-layer stack. The single layer film can be made ofdielectric materials as readily understood in the art. The multi-layerstack can comprise alternating layers of a low-index material likesilica and a higher-index material as readily understood in the art. Forinstance, the anti-reflection coating 240 may comprise the samedielectric material 230 filled in the resonant cavity. This isparticularly convenient for fabrication since the anti-reflectioncoating and the dielectric material filling may be deposited in a singlefabrication step. The anti-reflection coating 240 may have a thicknessfrom about 1 nm to about 200 nm depending on the desired performance ofthe selective absorber.

The selective absorber may also include a diffusion barrier layer 250disposed between the periodic structure and a substrate 260 to preventthe metal layer 220 from mixing with the substrate 260 either at hightemperatures or at room temperatures. The diffusion barrier layer 250may comprise two sub-layers 252 and 254. The first sub-layer 252 maycomprise SiN and the second sub-layer may comprise SiO₂. The firstsub-layer 252 can be selectively etched during fabrication and providesupport for the cavity walls.

Solar-Thermal Photovoltaic Systems with Metallic-Dielectric SelectiveAbsorbers

FIG. 3 shows a system 300 for converting solar radiation intoelectricity using a selective absorber like the one shown in FIG. 2. Thesystem includes a solar concentrator 310 to collect solar radiation 1over a larger area and focus it onto a smaller area of a wavelengthselective device (selective absorber) 320 to increase the electricalpower generated from each photovoltaic cell. The wavelength selectivedevice 320 absorbs the solar radiation below a cut-off wavelength andconverts it into thermal energy that is re-emitted as radiation in apredetermined wavelength band. Following the wavelength selective device320 is a photovoltaic cell 330 to receive the re-emitted radiation andconvert the radiation into electricity. The wavelength band of there-emitted radiation can be tuned to match the band gap of thephotovoltaic cell 330 in order to increase the conversion efficiency.

The solar concentrator 310 in the system 300 can increase the efficiencyand reduce the cost of solar power since more electricity is obtainedper photovoltaic cell. The solar concentrator can comprise variousoptical elements, including, but not limited to refractive lenses,diffractive lenses, spherical mirrors, parabolic mirrors, luminescentsolar concentrators, and any other suitable element. The system 300 caninclude more than one solar concentrator 310 to collect solar radiationfrom different directions, thereby covering a larger solid angle andfurther increasing the collection efficiency. This arrangement can alsoaccommodate the changing positions of the sun during the day.

In this example, the wavelength selective device 320 includes a metallicstructure 322 defining at least one resonant cavity 324 that is filledwith a dielectric material 326 (for example, as shown in FIG. 2). Ananti-reflection coating 328 is deposited on the metallic structure 322to reduce diffraction losses and increase absorption efficiency.

The metallic structure 322 can be periodic with a period less than thewavelength selective device's cut-off wavelength. The shape of theresonant cavity 324 defined by the metallic structure 322 may depend onthe desired optical modes or manufacturing constraints. In one example,the resonant cavity is cylindrical, which is convenient for computersimulation and practical fabrication. In other examples, the shape maybe elliptical, square, rectangular, triangle, polygon, etc. The shapemay even be irregular if a particular optical mode is desired.

The metallic structure 322 can have a surface reflectivity greater than70% for radiation at wavelengths of practical interest. But thereflectivity may be below 70% given that other parameters of themetallic structure allows for desired optical modes. Materials that canbe used for the metallic structure 322 include tungsten, ruthenium,platinum, silver, tantalum, copper, gold and titanium, among others.Non-metal materials, such as silicon, may also be used as long asdesired optical modes can be established.

The metallic structure 322 may comprise two layers as shown in FIG. 3(and described above with respect to FIG. 2): a periodic structuredefining a plurality of resonant cavities 324 and a layer of metal 325disposed on the cavity wall 327 of the resonant cavities. In oneembodiment, the periodic structure may function as the supportingstructure for the metal layer 325. The thickness of the metal layer inthis embodiment can be greater than the skin depth of the incidentradiation in the metal layer in order to define optical modes.

In another exemplary embodiment, the cavity wall 327 can define its ownoptical modes and alter the overall absorption spectrum of the entirewavelength selective device. In this embodiment, the thickness of themetal layer 325 may be less than the skin depth given that modessupported by the periodic structure itself can alter the absorptionspectrum to a desired state.

In operation, the dielectric material 326 inside the resonant cavity mayfulfill one or more of the following purpose: 1) adjust the refractiveindex of the resonant cavity so as to tune the optical modes inside thecavity; 2) allow wide incident angle operation of the selective absorberby reducing the period without changing the cut-off wavelength, which isusually a predetermined value in a STPV system; and 3) increase theoverall mechanical stability of the wavelength selective device. Thedielectric material 326 can be substantially transparent (forexample, >50% transmission through the entire thickness of thedielectric layer) to infrared radiation, having a refractive indexlarger than one, and having a thermal expansion coefficient (TEC)greater than that of the metallic structure. Materials that may be usedinclude, but are not limited to, HfO₂, SiO₂, TiO₂, Al₂O₃, TiN and otheroxide ceramics.

The wavelength selective device 320 can be designed with the cut-offwavelength as the reference. The radius of the resonant cavity 324 canlie in a wide range from about 50 nm to about 5 m to support desiredoptical modes in an STPV system. One can also adjust the refractiveindex inside the cavity to match at least one optical mode to thepredetermined cut-off wavelength. Adjustment of the refractive index maybe realized by using different dielectric materials 326, tuning thetemperature of the resonant cavity 324, applying an electric field overthe resonant cavity 324, or any other methods known in the art. Anotherpossible way to tune the optical modes to match a predetermined cut-offwavelength, based at least in part on Equation (1), may involveadjusting the skin depth of the metal layer 325. One may change the skindepth by using different metal materials, tuning the temperature of themetal layer to alter its resistivity, or any other means known in theart.

The depth of the cavity 324 may be greater than half of the overall sizeof the cavity, which is the sum of the inner diameter plus the thicknessof the cavity wall. A smaller depth is also feasible since otherparameters may compensate or balance the effects induced by a shallowdepth. The thickness of the metal layer 325 can be greater than the skindepth of the radiation of interest, but may be smaller as well since thecavity wall can also define its own cavity and alter the optical modesinside the wavelength selective device. Experimentally, a thicknessgreater than 50 nm can establish desired optical modes and achievedesired absorption spectrum. Designing the thickness of the cavity wall327 may take into account its mechanical role of supporting the metallayer 325 during fabrication. A practical range would be between 10 nmto 200 nm.

The overall size of the cavity in one example is selected to be lessthan a predetermined cut-off wavelength to support at least one opticalmode having a wavelength in match with the cut-off wavelength. Thecut-off wavelength can be, for example, a band gap wavelength of aphotovoltaic cell. A practical range of this cut-off wavelength can befrom about 0.5 m to about 3 μm. In another example, when the metallicstructure comprise a periodic structure, the overall size of the cavity,which may also be the period of the periodic structure, is selected tobe less than half of a predetermined cut-off wavelength to minimize thediffraction loss at large incident angles. In this example, theselective absorber can absorb incident radiation at an incident anglefrom 0° (normal incidence) to about 70° (close to grazing incidence).

The wavelength selective device 320 may also include an anti-reflectioncoating 328, which may comprise the same dielectric material 326 in theresonant cavity. This is convenient for fabrication since theanti-reflection coating and the dielectric material may be deposited ina single fabricate step. The anti-reflection coating 328 may have athickness from about 1 nm to about 200 nm depending on the desiredperformance of the selective absorber.

The photovoltaic cell 330 in the system 300 may come in various formssuch as crystalline silicon cells, thin film cells, multijunction cellsor any other suitable photovoltaic cells known in the art. Suitable thinfilm cells include but are not limited to Cadmium telluride, copperindium gallium selenide, GaAs, and silicon thin film cells. The band gapof the photovoltaic cell 330 can be approximately equal to the cut-offwavelength of the re-emitted radiation from the wavelength selectivedevice 320 in order to increase (if not maximize) thesolar-to-electricity conversion efficiency.

Processes for Fabricating Metallic-Dielectric Selective Absorbers

FIG. 4A to FIG. 4I illustrate a CMOS compatible method of fabricating aselective absorber as shown in FIG. 2 and FIG. 3. The method starts froma substrate 260 coated with a diffusion barrier layer 250 as shown inFIG. 4A. The diffusion barrier layer 250 may comprise two sub-layers 252and 254. In one example, the first sub-layer 252 may comprise SiN andthe second sub-layer 254 may comprise SiO₂. In FIG. 4B, a sacrificiallayer 270 is coated above the diffusion barrier layer 250. In FIG. 4C,the sacrificial layer 270 is patterned with a plurality of holes arrayedin a substantially periodic way at a period a. The period a of theperiodic array can be less than a predetermined wavelength of light. Inone example, the predetermined wavelength can be a band gap wavelengthof a photovoltaic cell and may lie in a range between 0.5 μm and 3 μm. Asupporting material is then deposited over the inner walls of the holesas shown in FIG. 4D. The deposition can be conformal so the supportingmaterial can form a layer of uniform thickness over the inner walls ofthe holes. FIG. 4E shows an anisotropic removal of the supportingmaterial that is on top of the sacrificial layer 270 and on the bottomof the holes so as to create openings 271 through which the sacrificiallayer can be etched. The next step, as shown in FIG. 4F, is toselectively remove the remaining portion of the sacrificial layer 270but not the supporting material in order to create a supportingstructure 210 defined by the layer of the supporting material. Thesupporting structure 210 has substantially the same shape of theplurality of holes because the deposition of the supporting material isconformal. On the supporting structure, a layer of metal 220 isdeposited to form a plurality of resonant cavities 280 (FIG. 4G). Thedeposition of metal can also be conformal in order to create uniformdimensions of the resulting resonant cavities 280. FIG. 4H shows anotherdeposition step, in which a dielectric material is deposited into thecavities, as well as any possible space between cavities. This step mayfurther include depositing a layer of dielectric material on theresonant cavities to form an anti-reflection coating (ARC) layer 240.Or, the ARC may comprise the same dielectric material in the cavities.In this case, the ARC layer 240 may be formed by intentionallydepositing an extra amount of dielectric material once the cavities havebeen filled. FIG. 4I shows a polishing step in which the surface of theselective absorber is polished. In one example (FIG. 4I), the polishingstep exposes the metal dielectric interface 290 directly toward incidentradiations. In another example, a layer of anti-reflection coating 240may be preserved.

The sacrificial layer 270 may comprise Poly Silicon, which may bedeposited on the substrate by a low pressure chemical vapor deposition(LPCVD) process. The thickness of the sacrificial layer may bedetermined by the desired depth of the resonant cavities in the finalphotonic crystal. For instance, the sacrificial layer may have a depthselected such that the depth of the resonant cavities is greater thanhalf the period of the periodic array of holes.

Determining the period a of the plurality of holes may be based, atleast in part, on a predetermined wavelength of light. For example, thepredetermined wavelength of light can be a band gap wavelength of aphotovoltaic cell. A possible range of the band gap wavelength can befrom about 0.5 m to about 3 μm. In one example, the period a may be lessthan the band gap wavelength to support at least one optical mode thatcan match the band gap wavelength. In another example, the period a isless than half of the band gap wavelength to reduce or minimize thediffraction loss at large incident angles. In this particularembodiment, the selective absorber can effectively absorb incidentradiation at an incident angle from 0° (normal incidence) to about 70°(close to grazing incidence).

The patterning of the sacrificial layer 270 can be achieved by SF₆ basedreactive ion etching (RIE) for Poly Silicon materials. A photoresist 420is employed to transfer the pattern on the resist to the sacrificiallayer 270. The pattern could be of any shape including, among others,round, square, rectangular, triangle or polygon. The shape may even beirregular if a particular optical mode is desired. In one exemplaryembodiment, a sacrificial layer of 500 nm thick Poly Silicon ispatterned via an optical stepper with a center wavelength of 369 nm. Inthe example shown in FIG. 4J, the photo-mask 410 has a squarecheckerboard pattern and each square 411 in the pattern has a length 412of 0.6 μm. Due to the resolution limit of the stepper and diffractioneffects, the square pattern resolves to circles 421 on the developedphotoresist with a minimum inter-circle spacing 422 of 100-200 nm asshown in FIG. 4K. This technique enables the formation of a photoresistwith feature sizes (minimum inter-hole spacing in this example) smallerthan the wavelength of the light (369 nm in this example) used forcreating the photoresist pattern. In operation, smaller inter-holespacing leads to a larger number of resonant cavities on a given pieceof wafer, thereby improving the absorption efficiency of the photoniccrystal and/or lowering the fabrication cost.

The supporting material in FIG. 4D may be Al₂O₃, which can beconformally deposited via atomic layer deposition (ALD). Note thatconformal deposition results in the supporting material being disposedon every surface of the patterned sacrificial layer, including the top,the bottom, and the side wall(s) of each hole. A Cl₂+BCl₃ based RIE maybe used for anisotropic etching, removing only the supporting materialon the top and bottom of the holes, while leaving intact the supportingmaterial on the side walls for further processing. The thickness of thesupporting material may be determined out of functional consideration:the supporting structure stands by itself in the step shown in FIG. 4Fand supports a metal layer 220 in the step shown in FIG. 4G. In oneexemplary embodiment, a layer of 40 nm Al₂O₃ is used to construct thesupporting structure.

The remaining portion of the sacrificial layer in FIG. 4F may be removedby XeF₂ gas phased etching, which reacts with Poly Silicon material butdoes not react significantly, if at all, with the Al₂O₃ supportingmaterial, thereby creating a free-standing supporting structure made ofthe supporting material.

Another ALD may be used to deposit the metal layer 220 over thesupporting structure 210 as in FIG. 4G. In another example, a sputteringprocess is employed to deposit a layer of tungsten or other metal overthe supporting structure. Furthermore, chemical vapor deposition (CVD)may also be used in this step. Materials that can be used includetungsten, ruthenium, platinum, silver, tantalum, copper, gold andtitanium, among others. However, for certain applications, non-metalmaterials such as silicon may also be used here if the reflectivity anddegree of difficulty in fabrication is acceptable. The thickness of themetal layer 220 can be greater than the skin depth of the wavelength ofinterest in order to define clear optical modes. If, however, thethickness of the metal layer 220 is less than the skin depth, thesupporting structure compensate and alter the absorption properties byallowing for modes within the cavities defined by supporting structure.In this case, by tuning the combination of the supporting structure andthe metal layer, desired absorption spectra can be achieved.

The dielectric material 230 can also be deposited into the resonantcavities by an ALD process. In one exemplary embodiment, a 36-hour ALDdeposition of HfO₂ is used to fill up the cavities. Other materials thatmay be used to fill the cavities include SiO₂, TiO₂, Al₂O₃, TiN or otheroxide ceramic known in the art. ALD deposition may results in excessdielectric materials in certain areas on top of the wafer. A chemicalmechanical polishing (CMP) may be used in this case to remove the excessmaterial, thereby creating a flat surface.

The fabrication method may further include an annealing process torefine the selective absorber and remove undesired gap modes so as toimprove the absorption properties. The annealing may be achieved byplacing the resonant cavities in an inert environment at hightemperature for an extended period of time. The inert environment may besubstantially void of oxygen to prevent oxidation of the metal and othermaterials. In one example, the inert environment comprises 95% Argon and5% Hydrogen. The temperature may be around 1000° C. and the extendedperiod of time may be around 24 hours.

FIGS. 5A-5F show images of a selective absorber during and afterfabrication according to the process illustrated in FIGS. 4A-4I. In FIG.5A, a photoresist is patterned with a square checkerboard photo-maskwith a period of 0.6 m. An inter-hole spacing of 200 nm is achieved viaa combination of exposure time and photoresist developing over-etching.FIG. 5B shows the device after the fabrication step in FIG. 4E.Sidewalls of Al₂O₃ and the remaining portion of the sacrificial layer ofPoly Silicon are shown. In FIG. 5C, the Poly Silicon has been removedvia XeF₂, leaving only the Al₂O₃ shell as the supporting structure. Thisimage shows the device after the fabrication step in FIG. 4F. In FIG.5D, approximately 80 nm of ruthenium has been conformally deposited viaALD to create a plurality of resonant cavities arrayed periodically onthe substrate. FIG. 5E shows a final selective absorber in which a thinlayer of the dielectric material is left on top of the resonant cavitiesto function as an anti-reflection layer. FIG. 5F shows another finalselective absorber in which excess dielectric material has been totallyremoved and the metal-dielectric interface is directly exposed toincident radiation.

FIG. 6A shows a fully fabricated 6″ wafer on which the selectiveabsorber is created according to the method described above with respectto FIGS. 4A-4I. The wafer has been diced into 1 cm×1 cm chips. FIG. 6Bis a SEM image of the cross-section of the selective absorber in one ofthe diced chips shown in FIG. 6A. The image is taken at a 420 angle tobetter show the structure of the device. FIGS. 6A and 6B alsodemonstrate that the employed fabrication method is wafer-scale, therebyallowing mass production of the selective absorber to meet high demandsin alternative energy sources.

Experimental Measurement of a Selective Absorber

As an example, a selective absorber is fabricated according to themethod described above using Al₂O₃ as the supporting material, Rutheniumfor the metal layer, and HfO₂ for the dielectric filling. The selectiveabsorber has a period of 780 nm. For each resonant cavity 200 (FIG. 2),the dimensions are as follows: radius r of the resonant cavity 200 is200 nm, depth of the cavity 200 is also 200 nm, thickness of the metallayer 220 is 80 nm, thickness of the supporting wall 210 is 40 nm,thickness of the anti-reflection coating 240 is 25 nm. FIG. 7 shows themeasured absorption spectrum of the selective absorber, demonstratingthe broadband absorption of the device across the majority of the solarspectrum (here, between about 0.25 m and about 1.3 m) along with a steepcut-off at a wavelength of about 1.65 m. Note the absorption is plottedas a function of photon energy. The cut-off occurs at a photon energy ofabout 0.75 eV, which corresponds to a cut-off wavelength of 1.65 m. Thecut-off wavelength is located at mode M1 of the resonant cavities. Theabsorption stays at around 80% at wavelengths below the cut-offwavelength (above the cut-off photon energy) but drops to below 10% atwavelengths above the cut-off wavelength (below the cut-off photonenergy).

Without being bound by any particular theory, the broadband opticalproperties of the selective absorber in the visible regime may be aresult of the combination of a high density of cavity modes and an ARClayer. The dielectric filling essentially red-shifts the frequencies ofthe high order cavity modes to create a high density of states in thevisible regime. Experimentally, this can be observed in the largernumber of peaks in the measured absorption spectrum, each of the peakscorresponds to an optical mode of the resonant cavity. The first twomodes, M1 and M2, are standard cavity modes; however, the third mode, M3supports a hybrid cavity and surface plasmon polaritons (SPP) mode. Thecoupling between cavity and SPP modes may also contribute to theincreased absorption in the M3 mode. As comparison, the absorptionspectrum of a flat ruthenium layer is also presented. The absorptionpercentage is much lower and the cut-off frequency is much less sharp.

Measured spectra at various incident angles are shown in FIG. 8,demonstrating the large acceptance angle of the selective absorber up to70°. As explained above, the dielectric filling in the resonant cavitiesdown-shifts the frequency of the low order modes to be below thediffraction threshold and thus improves the wide angle absorption. Thecavity modes in FIG. 7 remain relatively fixed in frequency as afunction of angle, which is a characteristic of the cavity modes.

The selective absorber was then placed at 1000° C. for 24 hours in a 95%Ar and 5% H₂ environment and measured again as shown in FIG. 9 where theabsorption peaks remain high, thus demonstrating the high temperaturestructural stability.

FIG. 10 shows the simulated absorption spectra as a function of the ARCthickness. As the ARC layer increases in thickness, the overlap betweenthe cavity modes and the ARC layer reflection spectrum red shifts. Thus,based on the application, different absorption spectra can be obtainedby simply varying the ARC layer. A global optimization of the cavitygeometry, ARC layer thickness, material thicknesses, and materials couldbe performed to create a desired absorption profile. The highsensitivity of ARC layer on the absorption spectrum may haveapplications beyond visible light. For instance, in the infrared andterahertz region of the electromagnetic spectrum, dielectric ARC layersmay be useful for chemical sensing and imaging applications, such asthose employed at airport security checks. In another example,dielectric ARC layers may be used for biosensors due to the change ofresonant frequency in the presence of biomolecules on the coating whenilluminated.

FIG. 11 demonstrates the effect of metal layer thickness m_(t) on theabsorption spectrum, where both the metal layer and the supporting layerthicknesses are varied to keep the inner radius constant. For metallayer thickness m_(t) greater than the skin depth, the absorptionspectra remain relatively constant, as seen for m_(t)>50 nm. In thisregime, the thickness of the Al₂O₃ s_(t) has no impact on the absorptionspectra since the fields do not significantly penetrate the metal.

Once the metal thickness is below the skin depth, as shown for m_(t)=25nm, the transmission of the fields through the metal lowers theabsorption spectra. In this regime, the thickness of the Al₂O₃ can alterthe absorption spectra by allowing for modes within the Al₂O₃ to occur.

Conclusion

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of designing and making the couplingstructures and diffractive optical elements disclosed herein may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes (e.g., of designing and making thecoupling structures and diffractive optical elements disclosed above)outlined herein may be coded as software that is executable on one ormore processors that employ any one of a variety of operating systems orplatforms. Additionally, such software may be written using any of anumber of suitable programming languages and/or programming or scriptingtools, and also may be compiled as executable machine language code orintermediate code that is executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

What is claimed is:
 1. A system for converting solar radiation intoelectricity, the system comprising: a solar concentrator to focus thesolar radiation; a wavelength selective device, in optical communicationwith the solar concentrator, to convert solar radiation focused by thesolar concentrator over a range of incident angles from about 0° toabout 70° and below a cut-off wavelength into heat energy and to reemitthe heat energy as radiation in a predetermined emission band, thewavelength selective device comprising: (i) a metallic structuredefining at least one resonant cavity, (ii) a dielectric materialdisposed within the at least one resonant cavity, and (iii) ananti-reflection coating deposited on the metallic structure; and aphotovoltaic cell, in optical communication with the wavelengthselective device, to convert the radiation emitted by the wavelengthselective device into electricity, wherein the predetermined emissionband is substantially within a band gap of the photovoltaic cell.
 2. Thesystem of claim 1, wherein the cut-off wavelength is about 1 μm to about5 μm.
 3. The system of claim 1, wherein metallic structure comprises: aperiodic structure defining a plurality of resonant cavities; and alayer of metal disposed on an inner surface of at least one resonantcavity in the plurality of resonant cavities.
 4. The system of claim 1,wherein: the metallic structure has a first thermal expansioncoefficient; and the dielectric material has a second thermal expansioncoefficient less than the first thermal expansion coefficient.
 5. Thesystem of claim 1, wherein the at least one resonant cavity supports atleast one optical mode having a wavelength substantially equal to thecut-off wavelength.